mysql where clause variable help

i've been told a loop is not the right way to do it, which is the way i've been doing it and it has worked. I assumed putting it in a foreach loop would accomplish looping the query for variable with multiple values.

e.g. testa and testb are both in the array, $key.
foreach($key as $value) {

//mysql query

}

How do i run a single query not in a loop that will have both 'testa' and 'testb' in the WHERE clause?

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